Buying Dry / Active Electrodes

edited January 2015 in Electrodes
Could someone recommend some off-the-shelf dry active electrodes for use with OpenBCI? Chip speaks ill of some cheap ones in another thread. Are better ones readily available, even if they cost more?
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  • HI baobab,

    I was looking for some dry electrodes also so I asked Conor,

    His reply was:

    "We chose wet electrodes because nobody has created a consistently effective dry electrode that can record a good signal from the scalp through hair. Therefore, because OpenBCI is a prototyping tool, we wanted to provide electrodes that we know will work (and record a quality signal) for the sake of EEG."

    I know that's not what you were looking for, but I hope it explains the issue a little more clearly as to why.

    Jason


  • I still don't know of any dry active electrodes that are easily purchased, but I'm still hoping that others might  come up with some.  Let's keep this thread alive!  Who knows of some dry active that can be purchased?!?

    Thanks,

    Chip
  • I've had some very good luck with the biopac Ag-AgCl dry electrodes (not active).   $15 for a pack of 10, and they clip to standard connectors.  

    Using these I am able to record very clear alpha from O2 using no preparation whatsoever on a modified Mindflex band using HC-06 serial bluetooth and OpenViBE software.


  • I did not know that there were *passive* dry electrodes.  Very interesting. 

    You said that they "clip to standard connectors", yet the Biopac page says that they require the use of the Biopac LEAD120 cables.  Is this true...do you need a LEAD120 for each one?

    Thanks,

    Chip
  • edited September 2014
    I had the same question as Chip.  I wonder why they didnt just use standard EKG / EEG (i.e. clothing size) snap connectors?  The Ag-AgCl posts I use in my saline sensors are standard snap size and thus can plug into inexpensive EKG touchproof snap leads (often sold in sets of 5 for about $20).  This LEAD120 is priced at $40 according to Biopac.

    This is a nice find however.  Cognionics uses a Ag-AgCl "comb" like plastic sensor tip that flexes with pressure.  And g.tec also uses a comb, gold in their case.  Other dry sensor systems I've run across also use this comb idea.  Perhaps this Biopac comb could be made to connect with a tiny PC board we come up with, using ideas from some of the published active sensor circuit papers that are out there.  I bet there is some tiny highly integrated sensor chip already out there.

    This comb from Biopac is much more comfortable looking than the "bed of nails" photos on the OpenEEG active sensor site.  :-)

    My hunch is that using this as a passive sensor, with no saline solution or conductive surface liquid / gel / paste interfacing to the skin --  that the impedance would tend to vary somewhat.  Based on how much pressure is on the points.  ECube is getting a decent signal, but then we'd also ask, how stable is that over the course of a half hour or hour long session?  Saline sensors stay moist and conductive for such time periods.

    William
  • For the record, I've found this:

    Although it doesn't mention Ag-AgCl I asked them and confirmed their electrodes are made of.

    I'm absolutely a newbie, actually this is my first BCI research, hope some experienced user can test them and share it!
  • Yellow, thanks.

    That link you provide is to their online shop area. Curious about the company I found this,

    http://www.floridaresearchinstruments.com/about/staff/

    So they look knowledgeable. I'm going to write them and ask about their experience using this Ag-AgCl comb in a dry fashion. Originally (previous post) I thought these came from Biopac. But since FRI's pricing is more reasonable (esp. the wires), perhaps it is the other way around.

    My guess is that the skin impedance would vary quite a bit using this in a dry fashion. Usually Ag-AgCl is always used with a paste, gel, or saline interface. But Cognionics has a similar approach, using active amplification and a flexible sensor. So perhaps with the really high impedance amp we have (1 GOhm), that might help compensate. Still the CMRR with such widely varying impedances might suffer.

    William
  • edited January 2015
    Unrelated to this forum topic, I've had communications with the guy from Florida Research Instruments...he's a very nice guy...he's really looking to help people out.  I think that I might buy a few of these to play with them.

    Chip
  • I just bought a set...a get their package deal with the matching lead wires...


    I still would love to find some active electrodes...dry or wet.

    Chip
  • edited January 2015
    I spoke with Karl Zurn at FRI. They are working on a couple products: both an elastic cap with 10-20 holes, and an active sensor board.

    Once you have a cap with holes in it, it is an easy process to thread a comb sensor through that from the inside, then clip on the electrode wire on the outside surface. Now the comb will self align to your head when you pull on the cap.

    Karl also points out these BraiNet 10-20 placement bands (mesh cap)
    would be another way to get the equivalent of a cap with hole positions.

    http://bio-medical.com/products/brainet-placement-caps.html

    [Note the holes on that BraiNet are possibly 9mm or so in diameter, so the comb sensors might squeeze through that hole. Solution would be to place a thin rubber sheet washer, say .75in diameter -- between back side of comb and the cap. Then attach lead.]

    While the comb electrodes will work well for hair areas, they would be uncomfortable directly on the forehead. In those positions you would use the other non-comb version.
  • I have been speaking with a number of people about
    Dry Electrodes.  I tried to explain some
    of the options.   Go to our web site www.FRI-FL.com and click on the Manuals
    tab.  Download EEG Manual.  This is a work in progress and it will be
    continually updated.

    Karl Zurn, Florida Research Instruments

  • We don't use a standard ECG cable because we want to be able to inject a gel (not a paste) into the electrode if necessary.  A typical snap ECG cable would cover up the hole.
    Karl Zurn, Florida Research Instruments
  • This showed up on the OpenBCI Twitter feed...new style of dry electrodes for ECG/EMG...maybe could be repurposed into EEG...


    ...I did not purchase the technical paper, so I cannot evaluate how new or different this is than other solutions.

    Chip
  • edited January 2015
    Found some interesting recent papers on dry EEG sensors, all from this Journal, devoted to Sensors,
    http://www.mdpi.com/journal/sensors

    2014 Review paper (covering many current dry sensors)
    http://www.mdpi.com/1424-8220/14/7/12847/pdf

    Searching that journal for papers on EEG,
    http://www.mdpi.com/search?q=eeg&journal=sensors&search=Search

    2014 Soft, Comfortable Polymer Dry Electrodes for High Quality ECG and EEG Recording
    http://www.mdpi.com/1424-8220/14/12/23758

    Abstract: Conventional gel electrodes are widely used for biopotential measurements, despite important drawbacks such as skin irritation, long set-up time and uncomfortable removal. Recently introduced dry electrodes with rigid metal pins overcome most of these problems; however, their rigidity causes discomfort and pain. This paper presents dry electrodes offering high user comfort, since they are fabricated from EPDM rubber containing various additives for optimum conductivity, flexibility and ease of fabrication. The electrode impedance is measured on phantoms and human skin. After optimization of the polymer composition, the skin-electrode impedance is only ~10 times larger than that of gel electrodes. Therefore, these electrodes are directly capable of recording strong biopotential signals such as ECG while for low-amplitude signals such as EEG, the electrodes need to be coupled with an active circuit. EEG recordings using active polymer electrodes connected to a clinical EEG system show very promising results: alpha waves can be clearly observed when subjects close their eyes, and correlation and coherence analyses reveal high similarity between dry and gel electrode signals. Moreover, all subjects reported that our polymer electrodes did not cause discomfort. Hence, the polymer-based dry electrodes are promising alternatives to either rigid dry electrodes or conventional gel electrodes.

    Image of the EPDM rubber combs from that paper:

    image


    See section 3.3.1 of that paper for how they put their tiny active board on the back of the EPDM combs; including schematic and more photos. Active board is a modified OpenEEG circuit!

    All the authors on that paper are from imec.be, here's an article on their site. Wonder if we could buy some? Image below shows the EPDM comb mounted on top of the active board.

    http://www2.imec.be/be_en/press/imec-news/imec-polymer-dry-electrodes.html

    image

    Here's a high rez image of their circuit board you can zoom into. Open in another tab in your browser, then zoom in.

    http://www.mdpi.com/sensors/sensors-14-23758/article_deploy/html/images/sensors-14-23758f4.png

    The TI chip on that tiny board is, http://www.ti.com/product/tlc2272


  • edited March 2015
    I believe these Imec EPDM combs were made through injection molding by the company that worked with Imec, Datwyler Group. The paper mentions some of the different conductive materials they added to the EPDM.

    However...  I also realized there are conductive filaments made for 3D printers. These are ABS or PETG based. Some using various forms of graphite or carbon black, another using carbon nanotubes.

    http://www.makergeeks.com/coabs3dfi10.html
    About 10K ohms/cm

    http://3dprint.com/3701/3dxtech-carbon-nanotube-3d-printer-filament/
    http://www.3dxtech.com/carbon-nanotube-filament/
    http://www.3dxtech.com/content/3DXNano_ESD_PETG_Data_Sheet_v1.pdf
    http://www.3dxtech.com/content/3DXNano_ESD_ABS_Data Sheet_v1.pdf
    From 10M to 1G ohms per ?

    These are of course plastic, not rubber. But with the 3D printing process you could make the comb teeth "fan out" in a way radially that would distribute the forces on the skin more gently than comb teeth that are strictly perpendicular, like the IMEC combs. In other words, the fanned out teeth would be more similar to the Cognionics combs. Since Joel @biomurph and Conor @conor_obci have the 3D printers right there, could be a way to experiment with comb sensors of varying form factors, possibly mated to active boards.

    While looking around the Cognionics site, I found a couple new pages I had not seen before,

    http://www.cognionics.com/index.php/products/hd-eeg-systems/quick-20-dry-headset


    image

    and

    http://www.cognionics.com/index.php/products/mini-systems/cap

    image

    William

  • If some OpenBCI  people want to try out EEG Dry Electrodes I
    can offer a special price for
    Package of
    25 Disposable / Reusable Dry Electrodes & 3 Lead Wires (FRI-2140-1) $29.95
    and this including shipping.  It is a
    savings of about $10.  They can be seen
    on our web site www.fri-fl.com  If anyone is interested send via PayPal $29.95
    to FloridaResearchInstruments@gmail.com
    with a note that it is for the OpenBCI Dry Electrode Offer.

    Regards,

    Karl Zurn

  • biomurphbiomurph Brooklyn, NY
    Hi Karl,
    Just sent you money. Thanks for the friendly discount!
    Can't wait to try them out!
    Joel
  • Hi Joel,

    The electrode package is in the mail.  I have some other packages (www.fri-fl.com) I
    would be happy to give OpenBCI people a discount on.  We have quite a few items in development and will
    keep you updated on our progress.  karlzurn@gmail.com

    Karl Zurn

  • edited February 2015
    Checkout some conductive filament 3D printed sensor combs that @conor_obci printed yesterday!

    http://conorrussomanno.com/2015/02/16/3d-printed-eeg-electrodes/

    Conor, I was looking around for the resistance in ohms of your BuMat filament, but did not easily find it. How does it compare to the 10K ohms figure mentioned on the Jan 29 post above? I guess one way to measure it is with an ohmmeter just between the back contact and say one of the front tips.


    image
  • Measuring the resistance of this stuff is not as easy as it appears on first glance.  It often has a sizeable surface resistance that complicates the measurement of its volume resistance.  So, one should really do a 4-wire resistance measurement vs the more common 2-wire measurement.

    When I did a 4-wire measurement across a variety of lengths, and normalized by the length measured and by the cross-sectional area, I got a volume resistivity of between 9-12 ohm-m (aka, 900-1200 ohm-cm).

    But, while doing this measurement, I noticed that the resistance measurements were not stable...they'd slowly creep upward as I left the probes on the sample.  This makes me think that there is some sort of capacitive component to this material that is clouding the DC-resistance measurement that i was doing.

    So, I tried an AC resistance measurement, where you use a function generator to drive an AC signal across your sample.  Continuing with the 4-wire approach, you measure both the current being injected and then you use a 2nd set of leads to measure the voltage across a known length of the sample.  Since the currents are down in the microvolt range (ie the resistance itself was in the multi-megaohm range), this is trickier than I'd like.  Using this technique, I got an answer that was about 2x more resistive (~20 ohm-m), but I had to do some compensation for the fact that impedance of my oscope probes were on the order of the resistance of my sample, so this AC measurement might not be any better that my DC measurement.

    So, I'd say that the resistivity of my batch of this material is on the order of 10-20 ohm-m.

    How's that for a complicated answer to a simple question?

    Chip
  • edited January 2016
    I was just browsing Karl Zurn's @KZurn website, FRI Florida Research Instruments. And see that they now sell a variant of the saline velcro headset idea, but adapted for the Ag-AgCl comb (dry or gel) electrodes mentioned earlier in this thread. Nice going, Karl and Brooks!

    image

    http://fri-fl-shop.com/product-category/electrode/electrodes/eeg-electrodes/

    And manual for use of the straps and electrodes. What they have done with the velcro straps is punch regularly spaced holes, that can mate with the comb or cone electrodes. This seems like a reasonable compromise with these particular electrodes which attach with a side-gripping spring jaw clamp. One potential downside is that the fixed hole spacing may not line up completely with the 10-20 site you are aiming for. But would likely be 'close'. Karl may want to add a comment here regarding their choice of hole spacing on the straps. (I'm sure which was chosen carefully.) The chin strap is elastic, but the Velcro One-Wrap straps are not.

    Karl, I wonder if have you also tried out your comb / cone sensors with the elastic band BraiNet straps. That would be another option that may provide closer 10-20 positioning. But I don't know if the holes are large enough on the BraiNet.

    image

    On Jeremy's @jfrey thread on the g.GAMMA cap printed electrode holders this also seems like a good system with pre-positioned 10-20 holes on an elastic / adaptive cap. So it would likely work with these dry combs as well.

    If Chip @chipaudette or Joel @biomurph have played with the FRI dry comb sensors, I wonder what they have found out relative to impedance and performance with and without gel injected.

    Finally, it might be possible to 3D print some kind of adapter with a post that would mate with the holes of the comb / cone sensors. Perpendicular to the post would be a flat plate that is glued to a small piece of One-Wrap. This could then provide arbitrary positioning of the sensors, as is done with the velcro saline system headset. But the geometry of this may be awkward given that there must be room for the spring clamp to attach. And it also precludes gel injection.

    William

  • I, too, have a bunch of those Ag/CL dry electrodes (bought from biopac), but without the combs. To build custom adapters and holders for those is on my TODO-list. I was planning to use once again 3D printing for the latter, but I have to say that the headband seems an easier solution for a first investigation.

    I'm definitely interested by some numbers about electrodes' performance if some have been quicker than me :)
  • In
    regard to the BraiNet cap the holes are too big for our dry electrodes but you
    can put one of our TD-22 Electrode Collars “washers” on the inside and they
    will work.  If anyone would like I to try
    these collars I could send a few at no charge. Email me at karlzurn@gmail.com   Also I could send a few of our dry
    electrodes.  These need one of our cables
    but you could uses an alligator to try them out.

    Karl
    Zurn, FRI  www.fri-fl.com

  • edited March 2015
    @wjcroft, @jfrey

    Dritz sew on snaps are a perfect fit for the comb Ag/AgCl electrodes.   I've made snap adaptors by soldering a Dritz size 10 to a Dritz size 4.

    image


    Here is a snapshot of signal quality between Fp1 and O1 using the electrode arrangement shown above.

    image
  • Wow, very nice catch!

    I think I'm gonna order some dritz product :D (except that the reference you point to are sold 21 bucks on amazon France :\)

    Just to be sure, the dritz snap to the electrodes, they are not soldered? Do you think it's possible do drill a hole in the dritz so we can still add gel later on?

  • @jfrey  If you look at the Amazon link, you'll see that there is a hole in the top just big enough to inject gel with a syringe ;-)   The Dritz snap is just a plain Nickel plated snap.   I created the leads you see in the photo using shielded wire soldered to a Dritz #4 snap, shrink tubing and velcro backing.   I used #4 so that I could use standard disposable ECG electrodes.  Here is a close up of the snap adaptors that I created for the comb electrode:  

    image
  • Thanks for the closeup!

    I wonder if the snap could damage the coating of the electrode after some time... But even with this caveat it seems one of the best solution to use those electrodes. I have the feeling that the ultimate EEG headset is one step closer :)
  • edited March 2015
    @jfrey I believe these comb electrodes are made of solid sintered Ag/AgCl, but that is a question for @KZurn.   :-)
  • edited March 2015
    Ecube, nice work with the button adaptor!

    Most Ag-AgCl buttons (and I think this comb) are plastic with a very thin silver plate coating, then chloride-ed. The 3D printed combs that Karl is experimenting with are also this plating process.

    The buttons I use with the velcro saline system are this way and snap into standard ECG female snap connectors. Generally since the buttons just stay in their sockets over many sessions (20 or so), they only get removed / reinserted a couple times. I do a rinsing / cleaning cycle about every 20 sessions. Then at about the 40 session mark, the chloride on the front of the button has started to wear off. So time to replace the button. But that's cool, because they only cost 50 cents each.

    The button posts do get slightly scratched by an insert / remove
    operation. But because of the rare remove operations, this is not an
    issue.

    Sintered Ag-AgCl disk electrodes that I've seen are very expensive. The Othmer Method ILF training uses these, they are supposed to be more stable than even the plated variety. Another down side to sintered is that they can only be cleaned in distilled water with USP grade salt. Any mineral impurities are said to degrade performance.

    ----

    An interesting development in the last couple years, has been the creation of a new type of dry passive electrode comb. The base material is a dense form of PU polyurethane, plated with Titanium, so the comb teeth have some flexibility. The outer surface is then processed into a nano-ized TiN, Titanium Nitride. The big neuro company ANT in NL is developing this. No products available yet that I'm aware. I have several papers they have published in this area if anyone is interested PM me. Here's a comparison between several dry passive types:  link here

    http://www.andreaproject.eu/

    http://www.ant-neuro.com/
  • We have introduced an EEG Headband.  I can offer the OpenBCI community a
    discounted price of  $59.95 Including
    shipping to the US.  Export add $5.  This is a $15 savings.  Send Payment via PayPal to
    FloridaResearchInstruments@gmail and mention this offer.

     

     FRI-2140-3    DRY EEG
    Dry/Wet Electrodes,  EEG Headband and 5
    Cables  (
    15 Disposable /
    Reusable Dry Electrodes and 10 EEG Cup
    Disposable/Reusable  Electrodes and 5 Cables– Red, White, Green, Black and Brown)  see our web site www.fri-fl.com under Products

    Karl Zurn, Florida Research Instruments

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